The invention relates to protection devices of the type comprising first and second differential transformers each having a toroidal magnetic core through which pass the phase conductors, and also perhaps, the neutral, of an alternating current distribution circuit, and a secondary winding, the secondary winding of the first transformer being coupled to the input of an amplifier followed by a rectifier, a threshold comparator and a control means suitable to actuate a circuit-breaker, whilst the secondary winding of the second transformer is coupled to the output of the said amplifier.
When, as a result of a direct contact with one of the phase conductors, a fault current circulates between this conductor and earth, the first transformer generates a voltage which, after amplification and rectification, is compared with a reference threshold in order to give a control signal which releases the circuit breaker after a certain delay. Further, as the neutral conductor is normally coupled to earth upstream of the protection device, if an insulation defect is produced accidentally downstream, the current which can then circulate in the feedback loop constituted by the neutral conductor and earth modifies the mutual inductance between the two secondary windings, so that the circuit constituted by these two windings and the amplifier can enter into oscillation. This oscillation at a relatively high frequency is rectified and compared with the reference threshold and will likewise release the circuit breaker. It is useful to observe that this protection is indispensable, when the neutral is distributed, so that the protection of persons shall not be prevented in the case of accidental earthing of the neutral conductor downstream of the device.
The protection devices of the type set forth above can in particular be mounted in current outlets intended to be installed in bathrooms or other locations where a protection is particularly necessary.